Polycarbonates are synthetic thermoplastic resins that may be derived from bisphenols and phosgene, or their derivatives. The desired properties of polycarbonates include clarity or transparency, high impact strength and toughness, heat resistance, weather and ozone resistance, and good ductility. They are useful for forming a wide variety of products, such as by molding, extrusion, and thermoforming processes. Branched polycarbonates, in some cases, can produce enhanced, or more desirable, characteristics over conventional linear polycarbonates. To form a branched polycarbonate, a branching agent, which has at least three functional groups, is added to the reaction of the dihydroxy compound and phosgene.
Many thermoplastic polymers require stabilization against discoloration from exposure to elevated temperatures. Exposure to high temperatures often arise during molding and extrusion processes. Fighting discoloration with heat stabilizers can create a dramatic increase in melt viscosity during heat aging. This can be detrimental to abusive processing and affect the end performance of the material. A specific issue may arise from the use of branching agents to produce branched polycarbonates is the resultant high residual content of ionic groups, such as chlorides. High chloride content can adversely impact melt stability and the color and/or transparency of articles molded from the polycarbonate.
Accordingly, there is a need for producing branched polycarbonate resins without sacrificing melt stability and the color or transparency of the resin or molded articles during a production process.